1. Field in the Industry
The present invention concerns a liquid-liquid contactor, particularly, improvements in liquid-liquid contactor in which mutually insoluble two liquids are continuously contacted in countercurrent flow due to the density difference between the two liquids. Typical use of the liquid-liquid contactor is liquid-liquid extraction and liquid-liquid reaction, which are important unit operations in the petroleum refinery, petrochemistry, coal chemistry, nuclear energy processing, and many other process industries.
2. State of the Art
Countercurrent flow type liquid-liquid contactors are divided into two types: one is non-agitated type such as perforated plate towers, packed towers, and baffle towers; and the other is mechanically agitated or pulsed type, such as rotary disk towers, Oldshue-Rushton towers, pulsed towers and reciprocal extraction towers.
Contactors of the latter type exhibit high contacting efficiency per unit height of the apparatus. The contactors of this type, however, have drawbacks that equipment investment is high and maintenance is troublesome, because they have mechanically driven parts. On the other hand, contactors of the non-agitated type are advantageous, though the contacting efficiency thereof is relatively low, because of lower investment and easy maintenance.
The inventors have continued research for improvements in the former type, non-agitated liquid-liquid contactors which can be installed with lower investment and operated with easy maintenance, particularly, in perforated plate towers having relatively high contacting efficiency.
Conventional perforated plate towers have the structure, as shown in FIGS. 11 and 12 (the Figures illustrate an embodiment in which light liquid "L" is the dispersed phase and heavy liquid "H" is the continuous phase), that a heavy liquid inlet (21) and a light liquid outlet (22) are provided at the top of a tower (20), that a light liquid inlet (23) and heavy liquid outlet (24) are provided at the bottom of the tower, and that plural trays (25) are distributed in the tower.
The tray (25) consists of a horizontal perforated plate (26) which is prepared by cutting off a part of a round perforated plate to provide a flow channel for liquid stream, and a vertical plate (27) which extends vertically downward (dispersed phase feeding direction) from the free end of the horizontal perforated plate. The flow channel formed by cutting off of the perforated plate and the vertical plate is the spout for overflow (the illustrated spout is called a "downcomer") or a channel for continuous phase liquid (29), through which only the continuous phase (heavy liquid) flows, and the apertures are channels for dispersed phase liquid (28) through which only the dispersed phase (light liquid) flows.
Operation of this perforated plate tower is carried out by feeding the light liquid from the light liquid inlet (23) at the bottom of the tower (20), and, at the same time, by feeding the heavy liquid from the heavy liquid inlet (21) at the top of the tower.
The two liquids are fed in such a manner that one of them forms a continuous phase, and the other, a dispersed phase. In the illustrated embodiment where the light liquid forms the dispersed phase, the light liquid is fed under the conditions that the flow rate of the light liquid to that of the heavy liquid forming the continuous phase is such a value that droplets of the dispersed phase accumulate and coalesce to a coalesced layer of the dispersed phase during staying under the perforated plates and moving upwardly, and that the coalesced layer of the dispersed phase flows out through the apertures or the channels for the dispersed phase liquid (28).
After having a component in the heavy liquid extracted with the light liquid, or contrarily, having a component in the light liquid extracted with the heavy liquid, or causing chemical reactions between the light and the heavy liquids by continuous and countercurrent liquid-liquid contacting of the light and the heavy liquids in the tower (20) as described above, the spent light liquid is drawn out through the light liquid outlet (22) and the spent heavy liquid through the heavy liquid outlet (24) continuously.
The above described perforated plate towers exhibit higher liquid-liquid contacting efficiencies per unit stage when compared with those of baffle towers having larger open areas of the channels for liquid streams. However, operation satisfying the above mentioned flow rate condition for the dispersed phase requires that the area of the channel for the dispersed phase liquid (28) is small or the apertures of the perforated plate are small, and therefore, feeding rate of the dispersed phase cannot be increased so much and the throughput of the liquids may not be enhanced.
In order to eliminate the above mentioned drawbacks the inventors performed the following experiments.
At first, they used a conventional perforated plate tower and increased the flow rates of the liquids at a constant ratio of the flow rates of the dispersed phase to the continuous phase until flooding occurs, and determined the liquid-liquid contacting efficiencies. The observed efficiencies were low in comparison with those under normal operations, and thus, it was concluded impossible to increase flow rate of the liquid to be treated while keeping the contacting efficiency high.
Then, they increased flow rate of the dispersed phase liquid by increasing open area of the channels for dispersed phase liquid by enlarging diameters of the apertures or by increasing the number of the apertures. This resulted in decreased thickness of the coalesced layer of the dispersed phase under the perforated plate (26), and in some occasion small changes in the operating conditions cause even disappearance of the coalesced layer of the dispersed phase. Thus, neither stable operation nor constant liquid-liquid contacting efficiency was possible. Further, it was observed that, in case where the coalesced layer of the dispersed phase disappears, the contacting efficiency becomes significantly low. The experiments suggested that developing this type of contacting is not hopeful.